Norovirus strain types found within the second infectious intestinal diseases (IID2) study an analysis of norovirus circulating in the community

Norovirus is the commonest cause of infectious intestinal disease (IID) worldwide. In the UK community incidence of norovirus has been estimated at 59/1000 population, equating to four million cases a year. Whilst norovirus infects people of all ages, a substantial burden occurs in infants and young children. The population of viruses found in sporadic cases among infants has been observed to be more diverse than that associated with outbreaks. In this study, we analysed norovirus-positive specimens collected during the second study of infectious intestinal diseases (IID2 Study) a national community cohort study conducted between April 2008 and August 2009 We examined the data for differences in circulating norovirus strains between two arms of a community cohort, and differences between genotypes and disease outcomes such as illness duration and symptom profiles.

Analysis was conducted to assess genetic diversity of noroviruses in the community. We also assessed differences in the cycle threshold (Ct) value, as a proxy for viral load, between norovirus genogroups and genotypes, and differences in reported symptoms or length of illness in relation to genogroup and genotype.

There were 477 samples where norovirus was detected. Whilst 85% of people recovered within two days for vomiting; diarrhoea symptoms were reported to day 4 for 83% of the cases, and 10% of people reported symptoms of diarrhoea lasting between five and six days. Both diarrhoea and vomiting symptoms lasted longer in children aged < 5 years compared to adults. There was a significantly higher proportion of GII.4 in samples obtained from the GP arm of the study (chi-square = 17.8, p < 0.001) compared to samples received via post in the self-reporting arm. In the latter group, the prevalence of GII.6 was significantly higher (chi-square = 7.5, p < 0.001).

We found that there is a difference in disease severity by age group. Children aged < 5 years had longer duration of illness, with 10% still having diarrhoea at seven days, and vomiting of between four and five days. The duration of illness reported is higher overall than one might expect for cases in the community in otherwise healthy individuals which has implications for infection control. No differences were observed in relation to duration of vomiting and or diarrhoea by genotype.